Passing an iterator to `bytearray.__init__` didn't take into account
that iterating that iterator could modify the bytearray position and
`ob_start`. When that happened the iterator would write beyond the end
of the allocation.
Update the "enough space" check to account for `ob_start`. The
assignment always included it. Add an assert after append that the
offsets line up as expected.
(cherry picked from commit 25181cee14)
Co-authored-by: Cody Maloney <cmaloney@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
